The invention relates generally to high-pressure plunger pumps used, for example, in oil field operations. More particularly, the invention relates to plunger packing and stress reduction in plunger pump housings.
Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. The fluid section usually comprises a housing which in turn comprises suction, discharge and cylinder bores, plus plungers, packing, valves, seats, high-pressure seals, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid section showing its connection to a power section by stay rods. A plurality of fluid sections similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid section design schematically illustrated in FIG. 2.
Each individual bore in a fluid section housing is subject to fatigue due to alternating high and low pressures which occur with each stroke of the plunger cycle. Fluid section housings typically fail due to fatigue cracks in one of the four areas defined by the intersecting suction, plunger, access and discharge bores as schematically illustrated in FIG. 3.
Among the designs proposed in the past for reducing pump housing fatigue failures in high-pressure fluid sections has been the Y-block housing design. The Y-block design, which is schematically illustrated in FIG. 4, reduces stress concentration in a fluid section housing by increasing the angles of bore intersections above 90xc2x0. In the illustrated example of FIG. 4, the bore intersection angles are approximately 120xc2x0. A more complete cross-sectional view of a Y-block plunger pump fluid section is schematically illustrated in FIG. 5.
Although several variations of the Y-block design have been evaluated, none have become commercially successful for several reasons. One such reason is that mechanics find field maintenance on Y-block fluid sections difficult. For example, replacement of plungers and/or plunger packing is significantly more complicated in Y-block designs than in the earlier designs represented in FIG. 1. In the earlier designs, provision is made to push the plunger distally in the cylinder bore, continuing out through an access bore labeled the suction valve/plunger cover in the illustration. This operation, which would leave the plunger packing easily accessible from the proximal end of the cylinder bore, is impossible in a Y-block design.
The Y-block configuration, while reducing stress in a fluid section housing, makes it necessary to remove the plunger from the proximal end of the cylinder bore. But because the proximal end of the cylinder bore is very close to the power section, plungers must be removed in two pieces. And even a two-piece plunger, schematically illustrated in FIG. 5, is itself a maintenance problem. The plunger pieces are often heavy and slippery, the connection between plunger pieces is subject to premature failures, and plunger pieces must be connected and disconnected in a confined space with limited visibility and accessibility. Nevertheless, the plunger pieces must be removed entirely from the cylinder bore in order to change conventional plunger packing.
A brief review of plunger packing design will illustrate some of the problems associated with packing and plunger maintenance in Y-block fluid sections. FIG. 6 is an enlarged view of the packing in an earlier (but still currently used) fluid section such as that illustrated in FIG. 1. In FIG. 6, the packing and packing brass are installed in the packing box of the fluid section. Note that packing brass is a term used by field mechanics to describe bearing bronze, where the bronze has the appearance of brass.
In the fluid section portion schematically illustrated in FIG. 6, the packing box is an integral part of the fluid section housing; it may also be a separate unit bolted to the fluid section housing. The packing is retained, tightened and adjusted by turning the gland nut. Removing the gland nut, however, does not allow one to remove the packing rings. Because packing rings must block high-pressure fluid leakage past the plunger, they are typically quite stiff, and they remain substantially inaccessible while the plunger (or any piece of it) remains in the cylinder bore. FIG. 7 schematically illustrates portions of a plunger pump housing and components including a gland nut and plunger parts, with the plunger pressure end within the packing box. Note, however, that the plunger pressure end cannot be rotated for removal until it clears the packing brass. This illustrates the necessity for a two-piece plunger in which the two pieces must be separated as they are individually removed from the cylinder bore.
The necessity for a multi-piece plunger in Y-block fluid section housings has not been eliminated by the recent introduction of packing assemblies such as those called xe2x80x9ccartridge packingxe2x80x9dby UTEX Industries in Houston, Texas. An example of such cartridge packing is schematically illustrated in FIG. 8. Note that removal of the gland nut exposes the packing cartridge housing, which in turn may be fitted with attachment means to allow extraction of the packing cartridge from the packing box (requiring proximal travel of the packing cartridge housing of approximately three to five inches).
This extraction, though, is not practical while a plunger piece lies within the packing box because of the excessive drag of the compressed packing rings on the plunger and packing box walls. Such compression can not be released unless all plunger pieces are removed from the packing box because the packing rings in the above cartridge packing assemblies are pre-compressed when the assemblies are manufactured. Further, any slight misalignment of apparatus used to extract such a cartridge packing assembly tends to cause binding of the (right cylindrical, i.e., not tapered) assembly within the (right cylindrical) bore. Analogous difficulties occur if an attempt is made to replace such a cartridge packing assembly while a plunger or part thereof lies in the packing box area. Hence, even if such cartridge packing assemblies were used in Y-block fluid section housings, multipiece plungers would preferably be used and field maintenance would be significantly complicated and expensive.
The invention comprises methods and apparatus to reduce or eliminate the above described problems of premature fluid section pump housing fatigue failure and difficult field maintenance related to plungers and/or plunger packing. Preferred embodiments of the invention may comprise either plunger pump housings having a conventional angular relationship among the plunger, suction and discharge bores, or pump housings having a Y-block configuration. In both a conventional angular relationship and a Y-block configuration, plunger, suction and discharge bore centerlines are substantially coplanar. But in a conventional angular relationship, the suction and discharge bore centerlines are substantially colinear, with the plunger bore (and an access bore, if present) at substantially right angles (i.e., angles equal to or nearly equal to 90 degrees) to both the suction and discharge bores. If an access bore is present, its centerline is preferably substantially collinear with the plunger bore. A plunger pump housing having such conventional angular relationships among bores is identified herein as having a right-angular configuration. In contrast, for plunger pump housings identified herein as having a Y-block configuration, the angle between the plunger bore and the suction bore, and/or the angle between the plunger bore and the discharge bore, is greater than 90 degrees.
In certain preferred embodiments of the invention, a Y-block or right-angular plunger pump housing comprises a suction valve bore, a portion of which has substantially circular cross-sections for accommodating a valve body and valve seat having substantially circular cross-sections. Note that the portion of the suction valve bore that accommodates a suction valve seat is preferably conical to facilitate substantially leak-proof and secure placement of the valve seat in the pump housing (e.g., by press fitting). Another portion of the suction valve bore comprises a transition area for interfacing with other bores. The suction valve bore circular cross-section has a first centerline. Bore centerlines are used herein to assist the reader in understanding how each bore in the fluid section pump housing is spatially related to other bores in the pump housing and other fluid section components.
A Y-block or right-angular plunger pump housing also comprises a discharge valve bore, a portion of which has a substantially circular cross-section for accommodating a valve body and valve seat having substantially circular cross-sections. Note that the portion of the discharge valve bore that accommodates a discharge valve seat is preferably conical to facilitate substantially leak-proof and secure placement of the valve seat in the pump housing (e.g., by press fitting). Another portion of the discharge valve bore comprises a transition area for interfacing with other bores. The circular discharge valve bore cross-section has a second centerline. The first centerline is preferably coplanar with the second centerline and either intersects it at a reference point (as in a Y-block housing), or is substantially colinear with it (as in a right-angular housing). The first and second centerlines may subtend a first obtuse angle (as in Y-block configurations), or an angle of about 180 degrees (as in right-angular configurations).
A Y-block or right-angular plunger pump housing further comprises a cylinder bore having a proximal packing area (i.e., an area relatively nearer the power section) and a distal transition area (i.e., an area relatively more distant from the power section). Between the packing and transition areas is a right circular cylindrical area for accommodating a plunger. The transition area of the cylinder bore facilitates interfaces with analogous transition areas of the suction valve bore and the discharge valve bore.
The cylinder bore packing area has a substantially circular cross-section for packing to slidingly seal against a substantially circular plunger within the bore. The packing and right circular cylindrical areas have a common (third) centerline. The third centerline is substantially coplanar with the first and second centerlines and preferably intersects them at or near the reference point (in the case of Y-block housings) or at a point about equidistant from the suction and discharge bores (in the case of right-angular housings). Thus, both Y-block and right-angular housings allow substantially unimpeded fluid flow from the suction bore to the discharge bore under the influence of reciprocating plunger movement in the cylinder bore.
In preferred Y-block configurations, the second and third centerlines subtend a second obtuse angle, and said first and third centerlines subtend a third obtuse angle. Preferred values for the first, second and third obtuse angles, as well as preferred intersections of the first, second and third bore centerlines, are determined primarily by design factors related to minimization of materials costs and/or machining costs.
In preferred right-angular configurations, the second and third centerlines subtend a right angle, and the first and third centerlines also subtend a right angle. The first and second bore centerlines are preferably collinear or, alternately, substantially parallel, and their intersection(s) with the third bore centerline is(are) determined primarily by factors such as those affecting materials costs and/or machining costs. Further applications of finite element stress analysis (FEA) analogous to those described herein may refine preferred design parameters related to centerline positioning.
In preferred embodiments of either Y-block or right-angular pump housing configurations, the transition areas of the suction, discharge, and/or cylinder bores comprise an elongated cross-section substantially perpendicular to each respective bore centerline. The long axis of each such elongated cross-section is substantially perpendicular to the plane of the first, second, and third centerlines.
Modem computer-aided FEA was used to study stress concentrations in the fluid section pump housing designs of the present invention and to document the stress-reducing effects of having one or more of the above elongated cross-sections in a plunger pump housing. Use of FEA thus made it possible to refine conventional Y-block pump housing designs to achieve surprisingly large stress reductions, and also to achieve nearly comparable (and similarly surprising) stress reductions in right-angular pump housings. While premature cracking had suggested the possibility of undesired stress concentrations in conventional (i.e., earlier) pump housings, the location, orientation and magnitude of these stress concentrations could not, as a practical matter, be adequately described without modem computers and FEA software. Early Y-block designs resulted in moderate reductions of premature cracking, but the lack of adequate stress descriptions prevented discovery and refinement of specific and efficient design changes for reducing stress, such as those of the present invention.
For example, FEA reveals that elongated cross-sections within the transition areas of the suction, discharge, and/or cylinder bores, as described above, are generally beneficial in reducing stress near the bore intersections. The shape of the elongations, however, may be optimized to obtain the greatest stress reduction. For example, while an elliptical cross-section is beneficial, an oblong cross-section is more beneficial.
The cross-section of an oblong bore consists of two opposing half-circles connected by substantially straight lines, which leaves a substantially flat portion between the cylindrical sections of the oblong bore. These substantially straight lines preferably have length between 5% and 95% of the length of radii of the opposing half circles. The unexpected result of incorporating one or more such oblong cross-sections within bore transition areas of a pump housing is that stresses in all areas of the intersecting bores of the housing are significantly reduced. Note that stresses are reduced in spite of the fact that pump housing material is removed and the fluid section side wall thickness is reduced in the area of each oblong cross-section. This material removal would ordinarily be expected to increase stress concentrations rather than reduce them.
An explanation of this surprising phenomenon lies in the role of the flat portions of each oblong bore. FEA analysis shows that stresses are dispersed along each such flat portion. Note that the adjacent flat portions of the transition areas of interfacing bores in the present invention are connected by relatively smooth surface transitions. Each such smooth transition is achieved by smoothing techniques known to those skilled in the art (e.g., chamfering and/or grinding to a predetermined radius). And each resulting smooth transition, termed herein a chamfer, effectively increases any discrete angles of intersections among the suction, discharge, and cylinder bores. Indeed, as used in the present application, a chamfer may preferably include a tapered portion of an oblong bore transition area to flare it out as it approaches a bore intersection, the transition from one bore to another thus being made even more nearly smooth. In contrast, earlier (completely circular) bores tend to concentrate stresses where they intersect with other circular bores, discrete angles of intersection being relatively smaller than in the present invention.
In addition to directly reducing stress concentrations in a pump housing, an oblong suction bore transition area of the present invention also simplifies certain pump housing structural features needed for installation of a suction valve with its spring and spring retainer. Specifically, a suction valve spring retainer of the present invention does not require a retainer arm projecting from the pump housing, nor are threads required to be cut in the housing to secure the suction valve. Benefits arising from the absence of a suction valve spring retainer arm include simplified machining requirements for the pump housing, and the absence of threads in the suction valve bore eliminates the stress-concentrating effects that would otherwise be associated with those threads.
Elimination of the suction valve spring retainer arm and certain pump housing threads is made possible in certain preferred embodiments of the present invention by use of spoked suction valve spring retainer ring or an oblong suction valve spring retainer. A spoked suction valve spring retainer ring, as discussed in the Detailed Description below, is inserted via, and retained within, the circular portion of a suction bore. An oblong suction valve spring retainer, in contrast, is inserted via, and retained within, an oblong transition area of a suction bore.
An oblong suction valve spring retainer comprises first and second complementary portions that can be clamped securely on either side of a lip projecting from the pump housing into a portion of a suction bore transition area having an oblong cross-section. Since installation of the oblong suction valve spring retainer, with its associated valve spring, valve body and valve seat, can be accomplished entirely from within a pump housing, no threads need be cut in the pump housing to secure the suction valve assembly. An added benefit of the oblong suction valve spring retainer of the present invention is that the retainer may comprise a self-aligning top stem valve guide assembly. Such a valve guide allows the use of top-stem-guided suction valves, a valve configuration that tends to reduce the adverse effects of both cavitation and flow resistance compared with other types of suction valves.
Another preferred embodiment of the present invention relates to a tapered cartridge packing assembly comprising a packing cartridge housing and related components. The packing cartridge housing has a distal end, a proximal end, a longitudinal axis, and a length between said distal and proximal ends. A substantially right cylindrical inner surface of the cartridge housing has a first diameter and, in certain preferred embodiments, a substantially coaxial right cylindrical outer surface extends distally from said proximal end for a portion of said cartridge housing length. In the latter preferred embodiments, a conically tapered substantially coaxial outer surface extends distally from said distal extent of said right cylindrical outer surface to said cartridge housing distal end, said tapered outer surface tapering distally from said right cylindrical outer surface toward said longitudinal axis.
The right cylindrical outer surface portion, when present, provides for consistent compression (i.e., adequate sealing) of O-ring seals associated with the cylindrical surface during longitudinal movement of a tapered cartridge packing assembly. The O-ring seals may be present in circumferential grooves on the outer cylindrical surface of such an assembly and/or in circumferential grooves on the corresponding inner cylindrical surface of a pump housing made to allow installation of the assembly. Such cylindrical surface portions are preferred for cartridge packing assemblies having conically tapered portions with tapers greater than about 1 degree. For conically tapered portions with tapers between about 0.5 and 1 degree, sealing via O-rings that may lie in one or more grooves on the tapered portion of a cartridge packing assembly (and/or that may lie in one or more grooves in the corresponding tapered surface of a pump housing) becomes less problematical. In such assemblies, the right cylindrical outer surface portion may be made relatively shorter or may be eliminated entirely because adequate O-ring compression for sealing between a cartridge packing assembly and a pump housing is maintained within a range of longitudinal assembly movement necessary for adjusting compression of the packing rings in these assemblies to obtain a sliding seal over a pump plunger.
The inner surface of the packing cartridge housing has a substantially coaxial cylindrical recess having a second diameter greater than said first diameter and extending from said distal end proximally to an internal stop. In certain preferred embodiments, the cylindrical recess has a substantially coaxial internal snap ring groove, said groove having a substantially uniform width and a third diameter greater than said second diameter.
There is at least one circumferential seal groove in said right cylindrical outer surface or, alternatively, in the inner surface of the portion of the pump housing into which a packing cartridge housing is inserted. An elastomeric seal is fitted within each said circumferential seal groove. A substantially coaxial bearing ring lies within the cylindrical recess; it has an inner diameter slightly less than said first diameter and an outer diameter about equal to said second diameter. The bearing ring contacts said internal stop. A substantially coaxial anti-extrusion ring also lies within the cylindrical recess. The anti-extrusion ring contacts said bearing ring. With an inner diameter slightly less than said first diameter and an outer diameter about equal to said second diameter, the anti-extrusion ring has a close sliding fit against a plunger in the cylinder bore, thereby effectively preventing extrusion of plunger packing proximally.
In certain preferred embodiments, a substantially coaxial snap ring having a thickness less than said snap ring groove width lies within the snap ring groove. The snap ring has an inner diameter slightly greater than said first diameter and an outer diameter slightly less than said third diameter, said snap ring having a longitudinal sliding fit within said snap ring groove. The snap ring, when present, aids in removal of certain components of a tapered cartridge packing assembly. But in embodiments having a gland nut integral with the proximal end of the packing cartridge housing, the snap ring may be eliminated.
A substantially coaxial packing compression ring has an inner diameter slightly greater than said first diameter and an outer diameter slightly less than said second diameter. When a snap ring is present, the packing compression ring has a thickness preferably greater than said snap ring groove width reduced by the snap ring thickness. The packing compression ring is positioned between said snap ring and said anti-extrusion ring and contacts said snap ring but is too thick to become lodged in said snap ring groove when the snap ring is in place in the groove. When a snap ring is not present, the packing compression ring is simply positioned distal to the anti-extrusion ring within the packing cartridge housing.
A substantially coaxial packing ring lies within said cylindrical recess. The packing ring has an inner diameter substantially equal to said first diameter and an outer diameter substantially equal to said second diameter. When a snap ring is present, the packing ring has sufficient length to substantially fill said recess between said anti-extrusion ring and said packing compression ring when said snap ring is positioned maximally distally within said snap ring groove. Note that proximally directed longitudinal sliding movement of said snap ring within said snap ring groove causes proximally directed longitudinal sliding movement of said packing compression ring with resultant compression of said packing. When, on the other hand, a snap ring is not present, the packing compression ring may still be caused to slide proximally, compressing the packing as described below.
A tapered cartridge packing assembly of the present invention is advanced distally into the tapered recess of the packing area of a cylinder bore of a plunger pump housing of the present invention through distal motion imparted by turning a threaded gland nut. The gland nut may be separable from the tapered cartridge packing assembly, but in an alternative preferred embodiment referred to above, the gland nut is integral with the proximal end of the packing cartridge housing (a tapered cartridge packing and gland nut assembly).
Before being advanced distally, the coaxial packing ring is uncompressed, which means that drag on a plunger which may be within the packing area of the cylinder bore is relatively low. But when a packing assembly comprising a snap ring is nearly fully inserted into the packing area (that is, within a distance from the end of its travel equal to the snap ring groove width), the snap ring encounters a coaxial cylindrical boss of the pump housing, the proximal face of which is termed the adjusting ring. Further (distal) advance of the packing assembly after the snap ring contacts the adjusting ring results in relative proximal longitudinal movement of the snap ring in its groove, with corresponding proximal movement of the packing compression ring. This proximal longitudinal movement of the packing compression ring results in compression of the coaxial packing ring with a consequent tightening of the packing around the plunger. Alternatively, when a packing assembly that does not include a snap ring is inserted into the packing area, the packing compression ring itself contacts the adjusting ring. Further (distal) advance of the packing assembly after such contact compresses the coaxial packing ring with similar tightening of the packing around the plunger.
Because of the shallow taper of at least a distal portion of its outer surface (preferably in the range of 0.5 to 3 degrees) and the circumferential elastomeric seal present in a groove on a proximal portion of that surface or within the cylinder bore, a tapered cartridge packing assembly will maintain an effective seal with a plunger pump housing during longitudinal sliding movement within the housing. When a snap ring is present, such movement is preverably less than or equal in magnitude to the snap ring groove width. Thus, as described above, the degree of tightening of packing around a plunger may be adjusted by varying the distance a packing assembly is advanced into a plunger pump housing of the present invention after the snap ring or packing compression ring contacts the adjusting ring. Note that during advance and withdrawal of a packing assembly, the tapered portion tends to maintain alignment with a cylinder bore, thus minimizing any tendency to bind.
Note also that distal advance of a tapered packing assembly or tapered packing and gland nut assembly of the present invention is preferably limited by the snap ring or, when the snap ring is absent, the gland nut shoulder, rather than by the assembly being wedged tightly into the tapered recess of a cylinder bore packing area. These complementary provisions to limit distal advance also act to minimize binding of the assembly in the tapered recess. Thus, withdrawal of a tapered packing assembly should be substantially free of binding while drag due to packing compression is substantially reduced as the assembly is withdrawn and the snap ring and/or the packing compression ring becomes free to move distally to relieve compression of the packing ring. These effects combine to make changing of packing with a plunger in the cylinder bore practical in the field.